The three line wires of a three-phase electric system, generally indicated by the numerals 1, 2, 3 or by the letters R, S, T, carry three sinusoidal electric alternating-current voltages, mutually shifted by a 120.degree. angle, are called "phases" of the system, and in the course of some technical operations it is necessary to be able to recognize, i.e. to single out, quickly and exactly said wires. Practically speaking, in many cases it is necessary to be able to state with absolute certainty which wire represents the phase "one", which the phase "two" and which the phase "three". In the present state of the art, two systems are mainly used to this end: one system utilizes a device composed of two gas-filled filament lamps with condenser or inductor, star-connected; another system employs a small asynchronous motor, previously calibrated, constituted by three circuits, of the voltmeter type, wound upon ferromagnetic cores arranged on a plane according to the apices of an equilateral triangle and electrically star-connected, between whose pole shoes an aluminum disc can rotate, which is subjected to the action of the rotating magnetic field produced by the three circuits.